Strongly Luminescent Pt(IV) Complexes with a Mesoionic N-Heterocyclic Carbene Ligand: Tuning Their Photophysical Properties

The synthesis, electrochemistry, and photophysical properties of a series of bis-cyclometalated Pt(IV) complexes that combine the mesoionic aryl-NHC ligand 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene (trz) with either 1-phenylpyrazole or 2-arylpyridine (C∧N) are reported. The complexes (OC-6-54)-[PtCl2(C∧N)(trz)] bearing cyclometalating 2-arylpyridines present phosphorescent emissions in the blue to yellow color range, which essentially arise from 3LC(C∧N) states, and reach quantum yields of ca. 0.3 in fluid solutions and almost unity in poly(methyl methacrylate) (PMMA) matrices at 298 K, thus representing a class of strong emitters with tunable properties. A systematic comparison with the homologous C2-symmetrical species (OC-6-33)-[PtCl2(C∧N)2], which contains two equal 2-arylpyridine ligands, shows that the introduction of a trz ligand leads to significantly lower nonradiative decay rates and higher quantum efficiencies. Computational calculations substantiate the effect of the carbene ligand, which raises the energy of dσ* orbitals in these derivatives and results in the higher energies of nonemissive deactivating 3LMCT states. In contrast, the isomers (OC-6-42)-[PtCl2(C∧N)(trz)] are not luminescent because they present a 3LMCT state as the lowest triplet.


■ INTRODUCTION
Transition-metal complexes featuring long-lived emissive triplet excited states are at the core of numerous technological, analytical, biomedical, and synthetic developments, including chemosensing, 1,2 cell imaging, 3 photodynamic therapy, 4 photocatalysis, 5 and light-emitting materials. 6−8 Over the past decades, most research in this area has focused on luminescent Ir(III) 8−13 and Pt(II) 14−18 complexes with cyclometalating heteroaromatic ligands because of the high tunability and adaptability of their excited states, whereas Pt(IV) complexes have only started to be systematically explored as strong emitters in recent years. 19 −27 In previous contributions, we have shown that several types of Pt(IV) complexes bearing cyclometalating 2-arylpyridines may exhibit efficient and long-lived luminescence from essentially ligand-centered triplet excited states ( 3 LC) that possess a very low metal-to-ligand charge-transfer (MLCT) admixture. 19,20,22,23,27 These characteristics make them promising candidates for applications that take advantage of relatively long excited-state lifetimes, such as sensing, singletoxygen sensitization, or photocatalysis. Although small, the extent of the MLCT contribution to the emissive state has been observed to fluctuate depending on the coordination environment, causing variations in the radiative rates. 21,22,28 Thus, shorter Pt−C bonds from metalated aryls or the presence of suitable π-donor ancillary ligands, e.g., the fluoride ion, result in occupied dπ orbitals with higher energies and greater MLCT admixtures, leading to higher radiative rates. 21 However, a more critical factor that influences the emission properties of cyclometalated Pt(IV) complexes is the presence of thermally accessible ligand-to-metal charge-transfer (LMCT) excited states originating from electronic promotions to dσ* orbitals, which can provide the effective nonradiative deactivation of the emissive excited state. Higher-energy LMCT states can be achieved by introducing strong σ-donor ligands, which usually lead to lower nonradiative rates and increased emission efficiencies. 23 N-Heterocyclic carbenes (NHCs) have emerged as very valuable ligands for the design of highly efficient luminescent complexes of late-transition-metal ions, such as Ir-(III), 29,30,39,31−38 Pt(II), 40,41,50,42−49 or Au(III). 51 The beneficial effects exerted by these ligands can be attributed to their exceptional σ-donor capabilities, 52,53 which lead to strong ligand−field splittings and an increased energy of nonemissive excited states that arise from electronic transitions to dσ* orbitals, which could otherwise become thermally populated and cause nonradiative deactivation or even degradation via ligand−metal σ-bond labilization. Consequently, the use of NHCs brings about improved stabilities and emission efficiencies, which are particularly important for the development of blue emitters. Diverse types of NHC ligands have been used to synthesize luminescent complexes, including chelating dicarbenes (C* ∧ C*), 54−56 pyridyl-NHCs (N ∧ C*), 57,58 and cyclometalated aryl-NHCs (C ∧ C*). 43,50,59 Most incorporate normal Arduengo-type NHC moieties, whereas the use of mesoionic NHCs is rather infrequent. 46,47,60−62 Although cyclometalating aryl-NHCs have been demonstrated as chromophoric ligands in homoleptic Ir(III) emitters, 37−39 mixed-ligand systems have also been developed in which they act as supporting ligands while other chelating heteroaromatic ligands, such as arylpyridines 34,63 or bipyridines, 32 are responsible for the emission.
We have recently developed a synthetic method that allowed the preparation of the first examples of Pt(IV) complexes bearing a cyclometalated aryl-NHC ligand. 64 The reported complexes combined a mesoionic carbene of the 1,2,3triazolylidene subclass (C ∧ C*) and either a monocyclometalating 2,6-diarylpyridine or a dicyclometalating 2,6-diarylpyridine (C ∧ N ∧ CH or C ∧ N ∧ C, respectively) and were found to display exceptionally intense phosphorescence in poly(methyl methacrylate) (PMMA) matrices at 298 K, which originated from a 3 LC state involving the C ∧ N ∧ CH or C ∧ N ∧ C ligand. In addition, the complex with a monocyclometalating C ∧ N ∧ CH ligand showed an intense luminescence in a fluid solution, which was marginally enhanced compared to that of similar bis-cyclometalated Pt(IV) complexes containing only C ∧ N ligands. However, no systematic evidence of the effects of supporting C ∧ C* ligands on the emission efficiencies of Pt(IV) complexes has been gathered so far.
In this work, we present a family of bis-cyclometalated Pt(IV) complexes bearing an aryl-1,2,3-triazolylidene ligand and cyclometalating C ∧ N ligands of different energies for the lowest π−π* transition, which exhibit strong phosphorescent emissions in fluid solutions and can reach quantum efficiencies of almost unity in PMMA matrices. Their emission properties are compared to those of bis-cyclometalated complexes bearing only C ∧ N ligands with the aim of providing a clear and general demonstration of the electronic effects of the C ∧ C* ligand.
The photoisomerization process can be easily followed by 1 H NMR spectrometry because the resonance of the proton ortho to the metalated carbon of the C ∧ N ligand is significantly shielded in the cis-isomers due to the diamagnetic current of the triazolylidene ring (e.g., 7.83 vs 6.31 ppm for trans-and cis-C,C*-2d, respectively). Further confirmation of the isomerization was provided by the X-ray diffraction study of complex cis-C,C*-2d ( Figure 1).  The treatment of cis-C,C*-2a−e with PhICl 2 led to the biscyclometalated complexes 3a−e, respectively, as the major products in all cases. However, the trichlorido complex (OC-6-41)-[PtCl 3 (C ∧ N)(trzH)] (4a−e) was also formed as a minor product (Scheme 1). This result differs from the reported reactions of the related cis-or trans-N,N-[PtCl(C ∧ N)(N ∧ CH)] complexes with PhICl 2 , which exclusively led to bis-cyclometalated Pt(IV) complexes. 19,24,66,67 Complexes 3 and 4 were obtained in different molar ratios depending on the C ∧ N ligand (Table S2). The most favorable outcome was obtained with the dfppy ligand in a 95:5 ratio (3b:4b), whereas the tpy ligand led to the lowest molar proportion of the biscyclometalated complex in a 60:40 ratio (3d:4d). The formation of these mixtures can be attributed to two competing processes that take place from the pentacoordinate Pt(IV) intermediate arising from the formal addition of a Cl + ion to cis-C,C*-2a−e (Scheme 2). The electrophilic metalation of the phenyl ring of the trzH ligand (path A) leads to 3, whereas the coordination of the Cl − ion released from the PhICl 2 reagent in the vacant coordination site produces 4 (path B). Apparently, the electrophilic metalation is less favored for the present triazolylidene ligand compared to that for N-coordinated 2-arylpyridines. The fact that derivative 3b was obtained in a higher molar proportion can be explained by the higher electrophilic character of the metal center in this case because of the diminished electron-donating ability of the dfppy ligand.
The isolation of complexes 3 from the above mixtures was only possible after several recrystallizations, resulting in low to moderate yields (from 13% for 3d to 50% for 3b). Complexes 4 could not be obtained in pure forms except for the tpy derivative 4d; nevertheless, the 1 H NMR spectra of enriched fractions allowed us to unequivocally establish their identities. In the case of the tpy derivative 4d, the considerable shielding of the proton ortho to the metalated tolyl carbon (6.71 ppm, J HPt = 33 Hz) indicates that it is directed toward the triazolylidene moiety and is affected by its ring current, implying that the mutual disposition of the tpy and carbene ligands is retained after the oxidative addition of PhICl 2 . In view of this configuration, we considered forcing the metalation of the phenyl group of the carbene ligand in complexes 4a and 4c−e at a high temperature in the presence of a base. Thus, by heating mixtures of 3 and 4 at 130°C in 1,2-dichlorobenzene in the presence of Na 2 CO 3 , complexes 4 produced the corresponding complexes 3, which could then be isolated in improved yields (38−64%).
The 1 H NMR spectra of complexes 3 corroborated the presence of two metalated aryl groups, each of which gives a considerably shielded resonance flanked by 195 Pt satellites arising from the proton ortho to the metalated carbon, which is affected by the diamagnetic current of an orthogonal ring. The crystal structures of 3d and 3e (Figures 2 and 3, respectively) are compatible with the NMR data, further confirming that the metalated aryls are mutually cis, while the carbene and pyridine moieties are trans to each other.
We also attempted the preparation of bis-cyclometalated complexes with a trans-arrangement of the carbene and aryl moieties to compare their photophysical properties with those of the isomeric complexes 3. The reaction of trans-C,C*-2a with PhICl 2 afforded a mixture from which complex (OC-6-42)-[PtCl 2 (ppz)(trz)] (5a; Scheme 3) could be isolated in a  23% yield, while the other products could not be identified. In the case of trans-C,C*-2d, the same reaction gave a mixture of the desired complex (OC-6-42)-[PtCl 2 (tpy)(trz)] (5d) and the trichlorido complex (OC-6-43)-[PtCl 3 (tpy)(trzH)] (6d) in a ca. 18:82 molar ratio; the mixture could be separated thanks to their different solubilities in MeOH, and the complexes were isolated in 11 and 67% yields, respectively. The 1 H NMR spectrum of complex 6d shows that the protons ortho to either the metalated p-tolyl or the coordinated N atom are not shielded by the triazolylidene ring, implying that the carbenic carbon is not coplanar with the tpy ligand. Reasonably, the cationic Pt(IV) intermediate complex isomerizes to avoid the trans-arrangement of the tolyl and triazolylidene groups, which explains the low yield in 5d. A similar result was previously found upon the oxidation of an analogous complex bearing monocyclometalating 2,6-di(p-tolyl)pyridine with PhICl 2 . 64 The crystal structures of 5a and 5d (Figures 4 and 5, respectively) confirmed the expected ligand arrangement. In both cases, the Pt−C* bond is nearly 0.1 Å longer than those in complexes 3 because of the high trans-influence exerted by the metalated aryl ring of the C ∧ N ligand. Additionally, significantly longer Pt−C1 bond lengths were found, e.g., 2.044 Å for 5d vs 2.011 Å for 3d, as a consequence of the higher trans-influence of the triazolylidene ring relative to that of the chlorido ligand.
Photophysical Properties. The electronic absorption spectra of 3a−e, 5a, and 5d were registered in a CH 2 Cl 2 solution at 298 K (Table 1 and Figure 6). Structured absorption bands are observed for complexes 3a−e in the 250−360 nm range that can be ascribed to essentially 1 LC transitions involving the cyclometalating ligands. 20,23,24 The lowest-energy band resembles those observed for complexes [PtMe(Cl)(C ∧ N) 2 ]; 23 its lowest maximum shifts from 320 to 354 nm along the sequence 3a → 3e in accordance with the decreasing energies of the lowest π−π* transition of the C ∧ N Scheme 3     Figure 6. Electronic absorption spectra of complexes 3a−e, 5a, and 5d in a CH 2 Cl 2 solution at 298 K.
ligands and can therefore be ascribed to a primarily 1 LC(C ∧ N) excitation. The absorption spectra of 5a and 5d differ from their respective isomeric complexes 3a and 3d mainly in regard to the lowest-energy feature, which appears to enclose additional absorptions. This is particularly evident for 5d, whose lowest-energy feature is significantly red-shifted with respect to that of 3d. We attribute these differences to LMCT transitions on the basis of TDDFT calculations (see below). The photoluminescence of 3a−e was examined in a deaerated CH 2 Cl 2 solution and poly(methyl methacrylate) (PMMA) matrices (2 wt %) at 298 K. Air-equilibrated samples were also examined to evaluate the luminescence quenching of these complexes by atmospheric oxygen. The obtained emission data are compiled in Table 2, and the spectra in the deaerated CH 2 Cl 2 solution are shown in Figure 7. In all cases, the excitation spectra match the corresponding absorption profiles ( Figure S29). The excitation and emission spectra in PMMA are almost identical to those in CH 2 Cl 2 ( Figure S30). In the absence of molecular oxygen, complexes 3b−e show intense emissions in both media, whereas 3a is not emissive in CH 2 Cl 2 and only weakly so in PMMA. In all cases, the bands are vibronically structured, and the energy of the highest-energy peak correlates with the triplet emission of the respective N ∧ CH ligand (ppzH, 378 nm; 68 dfppyH, 424 nm; 69 ppyH, 430 nm; 70 tpyH, 437 nm; 69 and thpyH, 485 nm 70 ).
Therefore, the C ∧ N ligand is the chromophoric one in all cases, while the cyclometalating trz acts as a supporting ligand. The radiative lifetimes range from tens to hundreds of microseconds, which is consistent with triplet emissive states of an essentially ligand-centered character ( 3 LC). The complexes bearing ppy-based ligands (3b−d) are the most efficient emitters, with quantum yields around 0.3 in solution that increase to almost unity in PMMA matrices; the latter are the highest values ever observed for Pt(IV) complexes. An analysis of their radiative and nonradiative rate constants (k r and k nr , respectively) shows that the large increases in the quantum yields in PMMA are a consequence of the inhibition of molecular motion, which leads to dramatic decreases in the value of k nr . The significantly weaker emission of the ppz derivative 3a can be attributed to the thermal population of a nonemissive 3 LMCT excited state because of the high energy of the 3 LC(ppz) state; a similar behavior has been reported for the tris-cyclometalated complexes fac-[Ir(ppz) 3 ] 37 and fac-[Pt(ppz) 3 ] + . 19 The lower efficiency of the thpy complex 3e compared with those of 3b−d can be explained by its lower emission energy, which must result in an increased nonradiative deactivation via a vibrational overlap with the ground state.
Air-equilibrated samples of 3b−d showed measurable luminescence, with quantum yields around 0.01 in CH 2 Cl 2 and 0.20 in PMMA. However, no emission could be detected from 3a in any medium, and 3e was emissive only in PMMA in the presence of atmospheric oxygen. Lifetimes dropped to a few microseconds in CH 2 Cl 2 . The calculated k r values in this medium remained the same order of magnitude as those obtained from deaerated samples, but k nr values increased by two orders of magnitude as a consequence of oxygen quenching. Much longer lifetimes were observed for samples in PMMA matrices; however, they could only be fitted to biexponential decays, which is probably because of the inhomogeneous oxygen distribution. The present data show that complexes of this kind could be used for luminescencebased applications in the presence of atmospheric oxygen as well as for the development of oxygen sensors.
For comparison purposes, the luminescence of     19 dfppy, 19 ppy, 66 tpy 19 and thpy 20 was also studied in a deaerated CH 2 Cl 2 solution and PMMA matrices (2 wt %) ( Table 3). These compounds show moderate or weak emissions except for the thpy derivative, which was not emissive in solution, and the ppz derivative, which did not show an emission in any medium. The observed emission spectra are almost identical in shape to those of the corresponding complexes 3, although they are slightly redshifted ( Figures S31 and S32). Single-exponential decays were observed for the ppy-based derivatives in the CH 2 Cl 2 solution, whereas double-exponential decays were obtained in PMMA.
Where possible, comparisons with complexes 3 show that lifetimes are significantly shorter for (OC-6-33)-[PtCl 2 (C ∧ N) 2 ]. In all cases, the measured quantum yields are much lower than those of complexes 3, implying that the replacement of one of the C ∧ N ligands by a cyclometalated trz mainly results in an enhancement of the emission efficiencies. This beneficial effect is primarily reflected in the k nr values, which are generally one order of magnitude lower for complexes 3 relative to the value of the corresponding (OC-6-33)-[PtCl 2 (C ∧ N) 2 ] complex, whereas variations in the value of k r are much less significant. Reasonably, the stronger σdonating ability of the carbene compared with that of the C ∧ N ligand pushes the metal dσ*-orbitals to higher energies in complexes 3, implying both that the deactivating LMCT states lie at higher energies and that their thermal population from the emitting state is more difficult than those in (OC-6-33)-[PtCl 2 (C ∧ N) 2 ] complexes, leading to lower nonradiative decay rates. In contrast to complexes 3, the isomeric 5a and 5d, where the carbene moiety is trans to the metalated aryl of the C ∧ N ligand, are not emissive in either the CH 2 Cl 2 solution or PMMA films at room temperature. A similar behavior was previously observed for the homologous unsymmetrical (OC-6-32)-[PtCl 2 (C ∧ N) 2 ] complexes (Chart 1), which was attributed to a thermally accessible and nonemissive 3 LMCT excited state that provides an effective nonradiative deactivation pathway. 21 Electrochemistry. The redox properties of the biscyclometalated complexes 3a−e, 5a, and 5d were examined by means of cyclic voltammetry in a MeCN solution. The voltammograms are depicted in Figure 8, and the potentials of the most important redox processes and highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO, respectively) energy estimations are compiled in Table 4. An irreversible oxidation peak was observed in the range from 1.71 to 2.03 V vs SCE except for the dfppy derivative 3b, where the oxidation must fall outside the accessible potential range. The associated HOMO energies vary according to the sequence 3c < 3a < 3d < 3e and agree with previously determined C ∧ Nbased π-orbital energies in cyclometalated Pt(IV) complexes. 19,20,27 The isomeric pairs 3a/5a and 3d/5d possess identical HOMO energies, suggesting that the HOMO is also primarily a π-orbital of the C ∧ N ligand in complexes 5a and 5d. The first reduction is irreversible for all complexes and is visible in the range from −1.54 to −1.69 V vs SCE for 3a−e, whereas for 5a and 5d it appears at distinctively less negative Chart 1 Table 3. Emission Data of C 2 -Symmetrical Complexes (OC-6-33)-[PtCl 2 (C ∧ N) 2 ]    19 it is likely that the LUMO in derivatives 3a, 3c, and 3d is a π*-orbital of the trz ligand, as predicted by the DFT calculations for 3d (see below). The LUMO energies found for 3b and 3e are compatible with a dfppy-and thpy-based orbital, respectively. In contrast, the significantly lower LUMO energies found for 5a and 5d imply that the LUMO is no longer ligand-localized in these complexes. Instead, it is assigned as a dσ*-orbital on the basis of DFT calculations (see below). In all cases, the first reduction is followed by a reversible or quasi-reversible wave in the E 1/2 range from −1.95 to −2.16 V vs SCE, and additional irreversible reductions were also observed. The reversible wave was observed at identical E 1/2 values for each of the pairs 3a/5a and 3d/5d, suggesting that the species produced as a consequence of the first irreversible reduction is not dependent on the ligand arrangement; however, its identity cannot be unambiguously established.
Computational Study. DFT and TDDFT calculations were performed for complexes 3d, (OC-6-33)-[PtCl 2 (tpy) 2 ], and 5d (see the Supporting Information for details). Frontier orbital energies and their main characters are presented in Figure 9. In the three cases, the HOMO is essentially a πorbital of the tpy ligand(s), with some contribution from metal dπ*-orbitals (4% for 3d, 7% for (OC-6-33)-[PtCl 2 (tpy) 2 ], and 3% for 5d). The LUMO and LUMO + 1 in 3d are π-orbitals of the trz and tpy ligands, respectively, and those in (OC-6-33)-[PtCl 2 (tpy) 2 ] correspond to π*-orbitals delocalized over the two tpy ligands. In contrast, the LUMO in 5d is essentially a dσ*-orbital that is mostly distributed along the N−Pt−Cl axis and lies at a noticeably lower energy with respect to the trzbased LUMO of 3d, which agrees with the electrochemical results. Notably, the lowest molecular orbital with a primarily dσ* character in complexes 3d and (OC-6-33)-[PtCl 2 (tpy) 2 ] is LUMO + 2, which has a significantly higher energy for the trz complex, implying that a major effect of the carbene is to increase the ligand-field splitting.
The TDDFT calculations reveal a ligand-to-ligand charge transfer (LLCT) from the tpy ligand to the trz ligand, π(tpy)−π*(trz), and a LC transition within the tpy ligand, π(tpy)−π*(tpy), as the lowest singlet excitations in complex 3d (S 1 and S 2 , respectively), whereas in (OC-6-33)-[PtCl 2 (tpy) 2 ] only LC(tpy) excitations are predicted to contribute to the lowest-energy absorptions. The three lowest singlet excitations in complex 5d are predicted to be weak and involve transitions from π(tpy), π(trz), or p(Cl) orbitals to dσ*-orbitals that can hence be designated as LMCT or LMCT/XMCT; a more intense excitation of primarily LC(tpy) character is predicted at a higher energy (S 4 ). Therefore, the presence of low-energy LMCT absorptions explains the red-shifted lowest-energy feature in the absorption spectrum of 5d.
The lowest triplet excitation energies are represented in Figure 10. The first triplet (T 1 ) corresponds to an essentially LC(tpy) transition in complexes 3d and (OC-6-33)-    In the case of 5d, the first triplet excitation is a LMCT transition, which explains the lack of emission of this complex. For further insight, a geometry optimization of the lowest triplet excited state (T 1 ) was carried out for the three studied complexes. The corresponding spin density distribution ( Figure 11) matches the topology of a π−π* excitation within the tpy ligand in 3d or one of the tpy ligands in (OC-6-33)-[PtCl 2 (tpy) 2 ], which is consistent with an essentially 3 LC(tpy) emitting state in these complexes, and the associated geometry variations relative to the ground state are mostly limited to the affected ligand (Table S13). The computed adiabatic energy differences with respect to the ground state are 2.78 eV for 3d and 2.75 eV for (OC-6-33)-[PtCl 2 (tpy) 2 ], which are a good match with the observed emission energies. The natural spin densities on the Pt atom are 0.0187 for 3d and 0.0221 for (OC-6-33)-[PtCl 2 (tpy) 2 ], indicating a small degree of metal orbital contribution and therefore a certain MLCT admixture in the essentially LC emitting state 21 that is slightly higher for (OC-6-33)-[PtCl 2 (tpy) 2 ]. This fact is consistent with both the increased metal orbital contribution to the HOMO in the latter complex and its lower emission energy that imply a higher energy of metal dπ orbitals, probably because the arylpyridine is a weaker π-acceptor than the cyclometalated trz. 52 In the case of 5d, the spin density distribution in the relaxed T 1 state clearly corresponds to a LMCT state involving an electronic transition to a dσ* orbital, which causes severe geometry distortions that mostly result from Pt−ligand bond elongations (Table S14).

■ CONCLUSIONS
Mixed-ligand Pt(IV) derivatives containing cyclometalating trz and C ∧ N ligands of different energies for the lowest π−π* transition have been synthesized by the oxidative addition of PhICl 2 to Pt(II) precursors of the type cis-or trans-C,C*-[PtCl(C ∧ N)(trzH)]. The electrophilic metalation of the pendant phenyl group of the trz ligand upon oxidation proved to be more difficult in comparison to analogous reactions involving 2-arylpyridines and compete with the coordination of a chlorido ligand. The complexes (OC-6-54)-[PtCl 2 (C ∧ N)-(trz)] that contain cyclometalating 2-arylpyridines (3b−e) exhibit strong phosphorescent emissions that originate from 3 LC states primarily localized on the C ∧ N ligand, which can reach quantum yields of ca. 0.3 in a fluid solution and almost unity in PMMA matrices; the latter are the highest efficiencies ever observed for Pt(IV) complexes. Therefore, they constitute a class of strongly emissive compounds whose emission energies can be tuned by varying the C ∧ N ligand. A comparison between the photophysical properties of 3 and those of the homologous C 2 -symmetrical complexes (OC-6-33)-[PtCl 2 (C ∧ N) 2 ] showed that the replacement of one of the C ∧ N ligands for trz results in lower nonradiative decay rates and higher quantum efficiencies. The computational results substantiate a higher energy of dσ* orbitals and deactivating 3 LMCT states in complexes 3, which are attributed to the strong σ-donor character of the trz ligand. In contrast, the isomeric complexes (OC-6-42)-[PtCl 2 (C ∧ N)(trz)] (5), featuring a trans arrangement of the carbene and aryl moieties, are not emissive because they present a 3 LMCT state as the lowest triplet, which involves a low-lying dσ* orbital along the N− Pt−Cl axis.

■ EXPERIMENTAL SECTION
General Considerations. Unless otherwise noted, procedures were performed at room temperature under atmospheric conditions using synthesis-grade solvents. Reactions involving silver reagents were conducted under a N 2 atmosphere in the dark. The dichloridobridged dimers 1a 71 and 1b−e, 28 PhICl 2 , 72 and the triazolium iodide salt 73 were synthesized following published procedures. NMR spectra were registered on Bruker Advance 300, 400, or 600 MHz spectrometers. Chemical shifts (δ) are given in parts per million downfield from tetramethylsilane. Elemental analyses were determined using a LECO CHNS-932 microanalyzer. The irradiation of trans-C,C*-2a was carried out using a 36 W Philips UVB Narrowband lamp centered at 310 nm. Complexes trans-C,C*-2b−e were irradiated with Blue LEDs following the previously described experimental setup. 74 General Procedure for the Synthesis of trans-C,C*-[PtCl-(C ∧ N)(trzH)] (trans-C,C*-2). The triazolium salt (100 mg, 0.29 mmol) and Ag 2 O (37 mg, 0.16 mmol) were suspended in CH 2 Cl 2 (10 mL), and the mixture was stirred for 14 h. The suspension was filtered through Celite, and [Pt 2 (μ-Cl) 2 (C ∧ N) 2 ] (1) (0.15 mmol) was immediately added to the filtrate. The mixture was stirred in the dark for 1 h and filtered through Celite. The filtrate was evaporated to dryness, and the residue was washed with Et 2 O (3 × 5 mL) and vacuum-dried to give trans-C,C*-2.
trans-C,C*-2a. White solid, obtained from 1a (109 mg). Yield: 100 mg, 58%. 1 13 13  General Procedure for the Synthesis of (OC-6-54)-[PtCl 2 (C ∧ N)(trz)] (3). To a solution of cis-C,C*-2 in CH 2 Cl 2 (5 mL) was added PhICl 2 , and the mixture was stirred for 30 min. Partial evaporation under reduced pressure (2 mL) and the addition of Et 2 O (20 mL) led to the precipitation of a pale-yellow solid, which was filtered off and vacuum dried. The obtained product was placed in a Carius tube with 1,2-dichlorobenzene (1 mL) and Na 2 CO 3 (30 mg), and the suspension was heated at 130°C for 16 h under a N 2 atmosphere. After cooling to room temperature, Et 2 O (10 mL) was added, and the resulting suspension was filtered. The collected solid was extracted with CH 2 Cl 2 (5 × 5 mL). The partial evaporation of the resulting solution under reduced pressure (2 mL) and the addition of Et 2 O (10 mL) led to the precipitation of a solid, which was filtered off and vacuum dried to give 3. (OC-6-54)-[PtCl 2 (dfppy)(trz)] (3b). In this case, a different synthetic procedure from the general method was followed. To a solution of cis-C,C*-2b (60 mg, 0.078 mmol) in CH 2 Cl 2 (5 mL) was added PhICl 2 (26 mg, 0.094 mmol), and the mixture was stirred for 30 min. Partial evaporation under reduced pressure (2 mL) and the addition of Et 2 O (20 mL) led to the precipitation of a white solid, which was filtered off, washed with MeOH (2 × 1 mL) and Et 2 O (2 × 3 mL), and vacuum dried to give 3b. Yield: 26 mg, 50%. 1

Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.